Fix part of #5070: Display empty answer message in drag & drop sort i…

…nteraction (#5323)

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## Explanation
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- Explain what your PR does. If this PR fixes an existing bug, please
include
- "Fixes #bugnum:" in the explanation so that GitHub can auto-close the
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  - when this PR is merged.
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Fixes part of #5070

Enables the `submit_answer_button` when the pending answer is empty.
Instead of disabling the button, an error message, stating "_**Arrange
the boxes to continue.**_", is now displayed when the user attempts to
submit without arranging the boxes.


https://github.com/oppia/oppia-android/assets/84731134/c0083266-8641-414a-b14d-de662123d4a2

## Essential Checklist
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#bugnum: ...".)
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---------

Co-authored-by: Adhiambo Peres <59600948+adhiamboperes@users.noreply.github.com>

app/BUILD.bazel

@@ -221,6 +221,7 @@ VIEW_MODELS_WITH_RESOURCE_IMPORTS = [
     "src/main/java/org/oppia/android/app/parser/StringToNumberParser.kt",
     "src/main/java/org/oppia/android/app/parser/StringToRatioParser.kt",
     "src/main/java/org/oppia/android/app/player/audio/AudioViewModel.kt",
+    "src/main/java/org/oppia/android/app/player/state/itemviewmodel/DragAndDropSortInteractionViewModel.kt",
     "src/main/java/org/oppia/android/app/player/state/itemviewmodel/DragDropInteractionContentViewModel.kt",
     "src/main/java/org/oppia/android/app/player/state/itemviewmodel/FractionInteractionViewModel.kt",
     "src/main/java/org/oppia/android/app/player/state/itemviewmodel/ImageRegionSelectionInteractionViewModel.kt",
@@ -320,7 +321,6 @@ VIEW_MODELS = [
     "src/main/java/org/oppia/android/app/player/state/itemviewmodel/ContentViewModel.kt",
     "src/main/java/org/oppia/android/app/player/state/itemviewmodel/ContinueInteractionViewModel.kt",
     "src/main/java/org/oppia/android/app/player/state/itemviewmodel/ContinueNavigationButtonViewModel.kt",
-    "src/main/java/org/oppia/android/app/player/state/itemviewmodel/DragAndDropSortInteractionViewModel.kt",
     "src/main/java/org/oppia/android/app/player/state/itemviewmodel/FeedbackViewModel.kt",
     "src/main/java/org/oppia/android/app/player/state/itemviewmodel/NextButtonViewModel.kt",
     "src/main/java/org/oppia/android/app/player/state/itemviewmodel/NumericInputViewModel.kt",

app/src/main/java/org/oppia/android/app/player/state/itemviewmodel/DragAndDropSortInteractionViewModel.kt

@@ -1,8 +1,10 @@
 package org.oppia.android.app.player.state.itemviewmodel
 
+import androidx.annotation.StringRes
 import androidx.databinding.Observable
 import androidx.databinding.ObservableField
 import androidx.recyclerview.widget.RecyclerView
+import org.oppia.android.R
 import org.oppia.android.app.model.Interaction
 import org.oppia.android.app.model.InteractionObject
 import org.oppia.android.app.model.ListOfSetsOfHtmlStrings
@@ -13,6 +15,7 @@ import org.oppia.android.app.model.SubtitledHtml
 import org.oppia.android.app.model.TranslatableHtmlContentId
 import org.oppia.android.app.model.UserAnswer
 import org.oppia.android.app.model.WrittenTranslationContext
+import org.oppia.android.app.player.state.answerhandling.AnswerErrorCategory
 import org.oppia.android.app.player.state.answerhandling.InteractionAnswerErrorOrAvailabilityCheckReceiver
 import org.oppia.android.app.player.state.answerhandling.InteractionAnswerHandler
 import org.oppia.android.app.player.state.answerhandling.InteractionAnswerReceiver
@@ -23,6 +26,18 @@ import org.oppia.android.app.translation.AppLanguageResourceHandler
 import org.oppia.android.domain.translation.TranslationController
 import javax.inject.Inject
 
+/** Represents the type of errors that can be thrown by drag and drop sort interaction. */
+enum class DragAndDropSortInteractionError(@StringRes private var error: Int?) {
+  VALID(error = null),
+  EMPTY_INPUT(error = R.string.drag_and_drop_interaction_empty_input);
+
+  /**
+   * Returns the string corresponding to this error's string resources, or null if there is none.
+   */
+  fun getErrorMessageFromStringRes(resourceHandler: AppLanguageResourceHandler): String? =
+    error?.let(resourceHandler::getStringInLocale)
+}
+
 /** [StateItemViewModel] for drag drop & sort choice list. */
 class DragAndDropSortInteractionViewModel private constructor(
   val entityId: String,
@@ -55,25 +70,34 @@ class DragAndDropSortInteractionViewModel private constructor(
       subtitledHtml.contentId to translatedHtml
     }
 
-  private val _choiceItems: MutableList<DragDropInteractionContentViewModel> =
+  private val _originalChoiceItems: MutableList<DragDropInteractionContentViewModel> =
     computeChoiceItems(contentIdHtmlMap, choiceSubtitledHtmls, this, resourceHandler)
 
+  private val _choiceItems = _originalChoiceItems.toMutableList()
   val choiceItems: List<DragDropInteractionContentViewModel> = _choiceItems
 
+  private var pendingAnswerError: String? = null
   private val isAnswerAvailable = ObservableField(false)
+  var errorMessage = ObservableField<String>("")
 
   init {
     val callback: Observable.OnPropertyChangedCallback =
       object : Observable.OnPropertyChangedCallback() {
         override fun onPropertyChanged(sender: Observable, propertyId: Int) {
           interactionAnswerErrorOrAvailabilityCheckReceiver.onPendingAnswerErrorOrAvailabilityCheck(
-            pendingAnswerError = null,
-            inputAnswerAvailable = true
+            pendingAnswerError,
+            inputAnswerAvailable = true // Allow submission without arranging or merging items.
           )
         }
       }
     isAnswerAvailable.addOnPropertyChangedCallback(callback)
-    isAnswerAvailable.set(true) // For drag drop submit button will be enabled by default.
+    errorMessage.addOnPropertyChangedCallback(callback)
+
+    // Initializing with default values so that submit button is enabled by default.
+    interactionAnswerErrorOrAvailabilityCheckReceiver.onPendingAnswerErrorOrAvailabilityCheck(
+      pendingAnswerError = null,
+      inputAnswerAvailable = true
+    )
   }
 
   override fun onItemDragged(
@@ -98,6 +122,7 @@ class DragAndDropSortInteractionViewModel private constructor(
     if (allowMultipleItemsInSamePosition) {
       (adapter as BindableAdapter<*>).setDataUnchecked(_choiceItems)
     }
+    checkPendingAnswerError(AnswerErrorCategory.REAL_TIME)
   }
 
   fun onItemMoved(
@@ -129,6 +154,20 @@ class DragAndDropSortInteractionViewModel private constructor(
       this@DragAndDropSortInteractionViewModel.writtenTranslationContext
   }.build()
 
+  /**
+   * It checks the pending error for the current drag and drop sort interaction, and correspondingly
+   * updates the error string based on the specified error category.
+   */
+  override fun checkPendingAnswerError(category: AnswerErrorCategory): String? {
+    pendingAnswerError = when (category) {
+      AnswerErrorCategory.REAL_TIME -> null
+      AnswerErrorCategory.SUBMIT_TIME ->
+        getSubmitTimeError().getErrorMessageFromStringRes(resourceHandler)
+    }
+    errorMessage.set(pendingAnswerError)
+    return pendingAnswerError
+  }
+
   /** Returns an HTML list containing all of the HTML string elements as items in the list. */
   private fun convertItemsToAnswer(htmlItems: List<StringList>): ListOfSetsOfHtmlStrings {
     return ListOfSetsOfHtmlStrings.newBuilder()
@@ -190,6 +229,13 @@ class DragAndDropSortInteractionViewModel private constructor(
     (adapter as BindableAdapter<*>).setDataUnchecked(_choiceItems)
   }
 
+  private fun getSubmitTimeError(): DragAndDropSortInteractionError {
+    return if (_originalChoiceItems == _choiceItems)
+      DragAndDropSortInteractionError.EMPTY_INPUT
+    else
+      DragAndDropSortInteractionError.VALID
+  }
+
   /** Implementation of [StateItemViewModel.InteractionItemFactory] for this view model. */
   class FactoryImpl @Inject constructor(
     private val resourceHandler: AppLanguageResourceHandler,

app/src/main/res/layout/drag_drop_interaction_item.xml

@@ -67,5 +67,12 @@
       app:draggableData="@{viewModel.choiceItems}"
       app:onDragEnded="@{(adapter) -> viewModel.onDragEnded(adapter)}"
       app:onItemDrag="@{(indexFrom, indexTo, adapter) -> viewModel.onItemDragged(indexFrom, indexTo, adapter)}" />
+
+    <TextView
+      android:id="@+id/drag_drop_interaction_error"
+      style="@style/InputInteractionErrorTextView"
+      android:text="@{viewModel.errorMessage}"
+      android:textColor="@color/component_color_shared_input_error_color"
+      android:visibility="@{viewModel.errorMessage.length()>0? View.VISIBLE : View.GONE}" />
   </LinearLayout>
 </layout>

app/src/main/res/values/strings.xml

@@ -453,6 +453,7 @@
   <string name="return_to_lesson">Return to lesson</string>
   <string name="explanation">Explanation:</string>
   <string name="drag_and_drop_interaction_group_merge_hint">If two items are equal, merge them.</string>
+  <string name="drag_and_drop_interaction_empty_input">Arrange the boxes to continue.</string>
   <string name="link_to_item_below">Link to item %s</string>
   <string name="unlink_items">Unlink items at %s</string>
   <string name="move_item_down_content_description">Move item down to %s</string>

app/src/sharedTest/java/org/oppia/android/app/player/state/StateFragmentTest.kt

@@ -747,6 +747,96 @@ class StateFragmentTest {
     }
   }
 
+  @Test
+  fun testStateFragment_loadDragDropExp_submitWithoutArranging_showsErrorMessage() {
+    setUpTestWithLanguageSwitchingFeatureOff()
+    launchForExploration(TEST_EXPLORATION_ID_4, shouldSavePartialProgress = false).use {
+      startPlayingExploration()
+      clickSubmitAnswerButton()
+      onView(withId(R.id.drag_drop_interaction_error))
+        .check(
+          matches(
+            withText(
+              R.string.drag_and_drop_interaction_empty_input
+            )
+          )
+        )
+    }
+  }
+
+  @Test
+  fun testStateFragment_loadDragDropExp_withGrouping_submitWithoutArranging_showsErrorMessage_dragItem_errorMessageIsReset() { // ktlint-disable max-line-length
+    setUpTestWithLanguageSwitchingFeatureOff()
+    launchForExploration(TEST_EXPLORATION_ID_4, shouldSavePartialProgress = false).use {
+      startPlayingExploration()
+
+      // Drag and drop interaction with grouping.
+      // Submit answer without any changes.
+      clickSubmitAnswerButton()
+      // Empty input error is displayed.
+      onView(withId(R.id.drag_drop_interaction_error))
+        .check(
+          matches(
+            isDisplayed()
+          )
+        )
+      // Submit button is disabled due to the error.
+      verifySubmitAnswerButtonIsDisabled()
+      // Drag and rearrange an item.
+      dragAndDropItem(fromPosition = 0, toPosition = 1)
+      // Empty input error is reset.
+      onView(withId(R.id.drag_drop_interaction_error))
+        .check(
+          matches(
+            not(isDisplayed())
+          )
+        )
+      // Submit button is enabled back.
+      verifySubmitAnswerButtonIsEnabled()
+    }
+  }
+
+  @Test
+  fun testStateFragment_loadDragDropExp_withoutGrouping_submitWithoutArranging_showsErrorMessage_dragItem_errorMessageIsReset() { // ktlint-disable max-line-length
+    setUpTestWithLanguageSwitchingFeatureOff()
+    launchForExploration(TEST_EXPLORATION_ID_2, shouldSavePartialProgress = false).use {
+      startPlayingExploration()
+      playThroughPrototypeState1()
+      playThroughPrototypeState2()
+      playThroughPrototypeState3()
+      playThroughPrototypeState4()
+      playThroughPrototypeState5()
+      playThroughPrototypeState6()
+      playThroughPrototypeState7()
+      playThroughPrototypeState8()
+
+      // Drag and drop interaction without grouping.
+      // Ninth state: Drag Drop Sort. Correct answer: Move 1st item to 4th position.
+      // Submit answer without any changes.
+      clickSubmitAnswerButton()
+      // Empty input error is displayed.
+      onView(withId(R.id.drag_drop_interaction_error))
+        .check(
+          matches(
+            isDisplayed()
+          )
+        )
+      // Submit button is disabled due to the error.
+      verifySubmitAnswerButtonIsDisabled()
+      // Drag and rearrange an item.
+      dragAndDropItem(fromPosition = 0, toPosition = 1)
+      // Empty input error is reset.
+      onView(withId(R.id.drag_drop_interaction_error))
+        .check(
+          matches(
+            not(isDisplayed())
+          )
+        )
+      // Submit button is enabled back.
+      verifySubmitAnswerButtonIsEnabled()
+    }
+  }
+
   @Test
   fun testStateFragment_loadDragDropExp_mergeFirstTwoItems_worksCorrectly() {
     setUpTestWithLanguageSwitchingFeatureOff()